On November 19, 2025, Health and Human Services Secretary RFK Jr. changed a section on the CDC website titled “Vaccines and Autism.” The website now includes the usual bogus claims about vaccines and autism that RFK Jr. has been promoting for the last 20 years. But the first statement is the most telling.
The website now states: “The claim ‘vaccines do not cause autism’ is not an evidence-based claim because studies have not ruled out the possibility that infant vaccines cause autism.” This statement takes advantage of a technicality in the scientific method that anti-vaccine activists have been using for years to promote fear of vaccines despite overwhelming evidence to the contrary.
Here’s how the scientific method works: Researchers interested in determining, for example, whether the MMR vaccine causes autism must first form a hypothesis: the null hypothesis. In this case, MMR does not cause autism. Upon completion, researchers can either reject or not reject that hypothesis. But they can never accept the hypothesis. They can never prove never.
Currently, 24 studies performed in seven countries on three continents involving thousands of children and costing tens of millions of dollars have all found the same thing. Children who have received the MMR vaccine were not at greater risk of developing autism than those who didn’t receive it. Researchers could have done 100 studies, or 1,000 studies, or 1 million but that still wouldn’t have proven that it couldn’t happen. You can never prove never.
If RFK Jr. wanted to be honest with the American public, he would make it clear on the CDC’s website that chicken nuggets also might cause autism, which has never been and will never be disproven.
However, at this point, it is fair to state that a truth has emerged: The MMR vaccine doesn’t cause autism; and neither does thimerosal, a vaccine preservative, or aluminum salts, a vaccine adjuvant.
Here’s another way to look at this. When I was a little boy in the 1950s, I watched the television show Superman, starring George Reeves. Superman flew. He looked down at the city below with his hands stretched in front of him (using an interlocking thumb grip) and, with his cape flying behind him, flew. When you are 5 years old, television does not lie. So, I went into my backyard, climbed onto a small chair with a towel tied around my neck to simulate a cape, put my hands in front of me (with the interlocking thumb grip) and tried to fly. Unsuccessfully. That didn’t prove that I couldn’t fly. I could have tried a million times, which also wouldn’t have proved it, each failed attempt only making it more statistically unlikely.
You can’t prove the absence of weapons of mass destruction in Iraq. You can only say that they were nowhere that you looked. You can’t prove that I have never been to Juneau, Alaska. (I have never been to Juneau, Alaska.) You can only show a series of pictures of buildings in Juneau with me not standing next to them. RFK Jr. uses a technicality in the scientific method to assure that no one can “prove” that he’s wrong. But he is wrong. Vaccines, probably the best studied of all environmental influences, have never been shown to cause autism. It is now fair to say that vaccines don’t cause autism. And that I can’t fly.
If RFK Jr. wanted to be honest with the American public, he would make it clear on the CDC’s website that chicken nuggets also might cause autism, which has never been and will never be disproven.
Paul A. Offit, MD, is director of the Vaccine Education Center and professor of pediatrics in the Division of Infectious Diseases at Children’s Hospital of Philadelphia. This piece originally ran on his Substack, Beyond the Noise!
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